The Public Switched Telephone Network (PSTN) is the backbone for providing telephony services to business and individuals in the United States. The PSTN includes a number of switches, generally designated as Service Switching Points (SSPs), for interconnecting a calling party's line to a called party's line. Prior to the 1960's, to complete a call between a calling party and a called party, signaling would occur over the trunk circuits between the switches to ensure that the called party was not busy and to establish a connection between the two parties. This earlier version of the PSTN was rather inflexible in that changes to the PSTN could only occur with the replacement of the hardware in the PSTN. For instance, at this time, the SSPs were hard-wired and had to be replaced with a new SSP in order to update the switch's capability. The switches, however, could not be quickly updated since the standards and specifications had to be well-defined for the various switch vendors. To address the delays in updating switches, these hard-wired SSPs were ultimately replaced with SSPs that had stored program control (SPC). As a result, rather than replacing an entire SSP, the SSP could be modified to enable a new feature simply by updating the software in the SSP. Even with SPC in the SSPs, the PSTN was still limited in the services that it could provide.
A major advancement to the PSTN occurred in the mid-1970's with the introduction of Signaling Transfer Points (STPs) and Signaling System number 7 (SS7) protocol. With the addition of SS7 and STPs to the PSTN, call setup information is routed over a signaling network formed between the STPs and no longer occurred directly over the trunks. For instance, a calling party's SSP would send a data query from one of its associated STPs to an STP associated with the called party. The called party's STP would then determine whether the called party's line was idle and would perform the necessary signaling over the SS7 data network to connect the call. Thus, whereas before call setup signaling would occur over the voice trunks, the STPs and SS7 signaling bypass this traffic away from the voice trunks and onto dedicated data lines. As a result, the capacity of the PSTN to carry voice calls was greatly increased.
In the mid-1980's, demand for additional services from the PSTN resulted in the Intelligent Network (IN). In general, IN provides service logic external to the SSPs and places this logic in databases called Service Control Points (SCPs). To accommodate IN, the SSPs have software to detect service-specific features associated with IN. The software in the SSPs define hooks or “triggers” for the services that require use of an SCP. In response to a trigger, an SSP queries an associated SCP for relevant routing information. For instance, IN permits 800 service and calling card verification service, both of which require a query from the SSPs to the SCP through an STP and the return of routing information to the SSP through an STP. A Service Management System (SMS) was also introduced into the PSTN with IN and provides necessary support in service creation, testing, and provisioning. The SMS communicates with the SCPs and provides software updates to the SCPs.
The demand for increased capabilities has more recently transformed the IN into an Advanced Intelligent Network (AIN). The AIN differs from the IN in that the AIN provides service independent capabilities whereas the IN was limited to service-specific capabilities. AIN provides a high level of customization and builds upon basic services of play announcement, digit collection, call routing, and number translation. Some examples of AIN services include abbreviated dialing beyond a central office, do not disturb service for blocking calls from certain numbers or at certain times, and area number calling service which allows a company to have one advertised telephone number but to have calls routed to a nearest business location.
The ability to provide Local Number Portability (LNP) is perhaps the latest enhancement to the PSTN. The local exchange carriers (LECs) are now required under the Telecommunications Act to provide local number portability so that subscribers can move or “port” their number from one service-provider to another service-provider. Traditionally, the function of a telephone number within the PSTN was both to identify the customer and to provide the PSTN with sufficient information to route a call to that customer. To allow a customer to change its service-provider while at the same time keeping the same telephone number, the telephone number can no longer by itself provide the means to inform the network of the customer's location. A database, called a LNP database, stores routing information for customers who have moved or ported to another local-service provider. The LNP database contains the directory numbers of all ported subscribers and the location routing number of the switch that serves them. With LNP, the SSPs will query an LNP database through a STP in order to correctly route calls to a ported telephone number.
The evolution of the PSTN from providing POTS to AIN services has primarily been driven by the need to support voice telephony. The PSTN, however, is not limited to voice telephony but is increasingly being relied upon for data services. Modems are the predominant means data is transmitted over the PSTN. The integration of voice services with data services is not a new phenomenon and the PSTN has traditionally accommodated these combined services through its Integrated Services Digital Network (ISDN) lines. An ISDN line can carry both voice and data traffic or can be optimized for data-only service at a speed of 128 kbps. Although the ISDN has been available for close to 20 years, the use of the ISDN line is not pervasive and estimates place the number of Internet subscribers employing ISDN service at only 1.4 percent.
Despite the infrequent use of ISDN service, the need for data services is quite extensive. The PSTN has been designed to carry a large amount of voice traffic with each voice call lasting, on average, just a few minutes. While an average voice call is approximately 3.5 minutes, the average Internet call lasts over 26 minutes. Considering that Internet traffic on the PSTN is soon expected to exceed the combined traffic of both voice and facsimile, the capacity of the PSTN will soon be stretched to its limits. The LECs have been meeting this higher demand for capacity by deploying additional switches and other elements within the PSTN. Unfortunately for the LECs, the cost of this additional PSTN equipment is being born almost entirely by the LECs since they will see little increase in their customer base. This added expense to each LEC is approximately $100 million per year and is thus a considerable expense to the LECs.
An immediate need therefore exists to alleviate strains on the PSTN due to Internet traffic. Some solutions to handle Internet congestion have been proposed in the Bellcore White Paper entitled Architectural Solutions To Internet Congestion Based on SS7 and Intelligent Network Capabilities, by Dr. Amir Atai and Dr. James Gordon. Many of these solutions discussed in this paper, however, require the design, development, and deployment of new network elements within the PSTN. For instance, several of the solutions introduce an Internet Call Routing (ICR) node which can perform SS7 call setup signaling and which is used to direct Internet calls to a data network. Other solutions rely upon a Remote Data Terminal (RDT) to alleviate congestion while other architectures propose the use of both ICRs and RDTs. The architectures described in the Bellcore White Paper are generally long-term solutions which offer limited assistance to the LECs in the near future. A need therefore still exists for systems and methods for addressing the ever-increasing amount of data traffic in the PSTN.